(1) Field of the Invention
The present invention relates to a refractory article known in the art as an “impact pad” for use in handling molten metals, especially steel. The invention particularly relates to an impact pad for placement in a tundish for reducing turbulence in a flow of molten steel entering the tundish. The present invention finds particular utility in the continuous casting of steel.
(2) Description of the Related Art
Tundishes act as holding tanks for said molten metal, and especially for molten steel in commercial processes for the continuous casting of steel. In the continuous casting of steel, the molten steel fed to the tundish is generally high-grade steel that has been subjected to various steps for rendering it suitable for the particular casting application. Such steps normally involve, for example, one or more steps to control the levels of the various elements present in the steel, for example the level of carbon or other alloying ingredients, and the level of contaminants such as slag. The residence of the steel in the tundish provides a further opportunity for any entrained slag and other impurities to segregate and float to the surface where they can be, for example, absorbed into a special protective layer provided on the surface of the molten steel. Thus the tundish can be used to further “clean” the steel before it is fed to the mould for casting.
To optimize the ability of the tundish to continuously furnish a supply of clean steel to the mould, it is highly desirable to control and streamline the flow of steel through the tundish. Molten steel is normally fed to the tundish from a ladle via a shroud that protects the stream of steel from the surrounding atmosphere. The stream of molten steel from the ladle generally enters the tundish with considerable force, and this can generate considerable turbulence within the tundish itself. Any undue turbulence in the flow of molten steel through the tundish has a number of undesirable effects including, for example; preventing slag and other undesirable inclusions in the steel from agglomerating and floating to the surface; entraining into the molten steel a part of the protective crust that forms, or is specifically provided, on the surface of thereof; entraining gas into the molten steel; causing undue erosion of the refractory lining within the tundish; and generating an uneven flow of the molten steel to the casting mould.
In an effort to overcome these problems the industry has undertaken extensive research into various designs of impact pads for reducing turbulence in the tundish arising from the incoming stream of molten steel, and for optimizing the flow within the tundish to approximate ideal “plug flow” characteristics as nearly as possible of the molten steel as it traverses the tundish. Generally speaking it has been found that the flow of molten steel through the tundish can often be improved using impact pads that have specially designed surfaces capable of redirecting and streamlining the flow of molten steel.
Plug flow behavior (i.e., passage of successive portions of steel through the tundish without significant mixing) requires direction of flow away from the tundish outlet after the molten steel recedes from the impact pad. The presence of a significant portion of flow from the impact pad to the tundish outlet, with a minimized residence time in the tundish, is known as “short-circuiting.” Impact pads disclosed in the prior art have generally been designed with particular attention to the upwardly directed component of the resulting flow. An increase in the residence time, and an increase in the uniformity of residence time, in the tundish corresponds to the minimization of mixing, and enables successive steel formulations to pass through the tundish with retention of their respective compositions.
Impact pads disclosed in the prior art generally comprise a base against which a downwardly directed stream of molten steel impinges, and a vertical sidewall or sidewall elements that redirect the stream. They are fabricated from refractory materials capable of withstanding the corrosive and erosive effects of a stream of molten steel for their working lives. They are frequently shaped in the form of shallow boxes having, for example, square, rectangular, trapezoidal or circular bases.
It will be appreciated that the process of designing a new tundish impact pad which meets particular pre-determined criteria is extremely complex, since changing one aspect of the design of an impact pad generally has unforeseen ramifications on the flow dynamics of the entire tundish system.